1. Field of the Invention
This invention relates to pigments for ink recording medium coating compositions, especially adapted for inkjet printing, and methods of making and using the pigments and coating compositions.
2. Description of the Related Art
Inkjet recording processes represent one of the most important and widely used technologies for high-speed electronic printing. Inkjet printers typically include a plurality of nozzles connected to a supply of liquid based ink. The nozzles can be energized to spray ultrafine liquid droplets of the ink upon demand. Typically, a series of the nozzles is controlled to emit the droplets of ink in the pattern of characters or images on a paper surface. Thermal bubble and piezoelectric printers are the two prevailing primary inkjet technologies currently used by printer manufacturers. Conventionally, inkjet printers have used an aqueous-based ink. Typical inks contain a minor amount of ink pigment and a major amount of water as a vehicle.
The paper used on an inkjet printer greatly determines the quality of the image printed. Papers suitable for inkjet printing typically involve a base paper coated with a composition that improves the ink reception properties of the paper. Base paper for ink receptive coatings is generally made from bleached chemical pulp to which fillers, dyes, and if needed, sizing agents and strength enhancers are added. The conventional ink receptive coating composition applied to the base paper generally includes a binder and porous fine powder capable of absorbing ink coated on a paper surface. Matte and high gloss ink jet papers are currently available at a significant price premium over uncoated papers.
Brightness and absorption properties of paper greatly affect image quality. Standard uncoated papers generally are not suitable for high-resolution inkjet printing. A rough or course paper scatters light in more directions than a smoother surfaced paper. The smoother paper makes images printed thereon appear brighter all other factors being equal. Regarding absorption, ideally, when the ink is sprayed onto the paper, it will stay in a tight, symmetrical dot. The ink should not be absorbed to deeply by the paper because the sprayed dot will loose optical density at the paper surface and tend to “feather.” “Feathering” means the sprayed dot of ink is absorbed by the paper in a manner such that it spreads out laterally in an irregular manner to cover a slightly larger area than intended. As a result, the printed image looks somewhat fuzzy, especially at the edges. High quality inkjet paper ideally would be precoated with a film that keeps the ink close to the paper surface to give a printed image of enhanced optical density, while permitting the aqueous medium or vehicle to be absorbed further into the body of the paper to accelerate setting and drying of the ink. This supports faster print rates and reduces set-off or ink transfer problems created by low vehicle absorbency. Therefore, improved print quality and accelerated ink drying times are desired. A proper balance of these properties is difficult to achieve, especially at higher printer resolutions and smaller dot diameters.
Currently silicas are the most widely used coating pigment for making coated inkjet paper. Structured silicas are synthetic products. The silicas generally create an acceptable balance of inkjet printability and ink drying attributes. However, the prior use of silicas for this purpose has drawbacks. Silicas are relatively costly to manufacture. In addition to the relative high cost of silicas, mill dusting associated with silicas during coating make down must be dealt with as a material handling issue. Furthermore, silicas have high surface areas, and coatings containing them tend to develop viscosity very rapidly with small increases in silica content. Consequently, for inkjet paper application coatings using silicas, the silica solids content typically is formulated to a relatively low value, while relatively large amounts of binder are required to achieve a sufficiently high binding strength. The increased coating viscosities encountered at lower pigment levels associated with the use of silicas as the absorptive pigment in paper coatings makes it difficult to manipulate coat weights at these low solids levels. The high surface area of the silica is useful in that it creates an open structure in a continuous binder phase. This open structure permits fast absorption of the ink leading to good ink drying properties on an ink jet printer.
U.S. Pat. No. 4,478,910 describes ink jet recording paper comprising a base sheet with a specific sizing degree having a coating layer comprising fine silica particles and a water-soluble polymeric binder.
U.S. Pat. Nos. 6,140,406 and 6,129,785 describe a coating composition for an inkjet recording medium comprising an aqueous suspension of absorptive silica pigment, polyvinyl alcohol binder, and a cationic fixing agent. The pigment preferably is a mixture of 75% or more silica gel having a pore volume of 0.5–2.0 cc/g, and 10% or more of alumina or alumina trihydrate.
U.S. Pat. No. 5,985,424 describes a coated paper for inkjet printing in which a base coat having good absorbency for ink vehicle and a top coat is an ink receptive coating. In a preferred embodiment the base coat contains a mixture of precipitated calcium carbonate and calcined clay dispersed in a standard coating binder, while the topcoat includes fumed or pyrogenic silica dispersed in an emulsion prepared from styrene polymerized in the presence of polyvinyl pyrrolidone (a non-standard binder).
Other types of pigments besides silica have been proposed for coating compositions for paper. For instance, conventional calcium carbonate powders used as a paper coating pigment do not functionally improve print characteristics of inkjet-coated papers. Consequently, conventional calcium carbonate can be beneficially added to paper coatings to impart optical effects, e.g., to enhance brightness and smoothness, but typically adversely affects printability and ink drying properties.
U.S. Pat. No. 6,441,076 describes production of a paper coating composition applicable to inkjet paper in which the composition contains a high solids level of ultrafine particle size calcium carbonate and dissolved fine particle size, partially hydrolyzed, low molecular weight polyvinyl alcohol. In this application the surface area of the calcium carbonate is very high in order to mimic the performance of a high surface area silica. Still color reproduction and ink drying are not consistent with a silica coated sheet.
U.S. Pat. No. 5,397,619 describes an ink jet recording paper comprising a base paper having a recording layer on at least one surface containing at least 40 weight % of a pigment and not more than 60 weight % of binder, having a surface roughness by ten point height on the recording layer surface of no more than 5 μm and an air permeability of nor more than 1,000 seconds. The pigment can be silica, white carbon or silica gel obtained by wet method, superfine silica obtained by dry method, or a calcium carbonate silica complex having a particle structure consisting essentially of silica crystallized in calcium carbonate crystals.
U.S. Pat. No. 6,274,226 describes mesoporous silicoaluminate pigments, which are formulated with polyvinyl alcohol as binder, for use in ink jet and carbonless paper coatings.
U.S. Pat. No. 5,997,625 describes a coating pigment for ink jet printing comprising hydrous clay, a caustic leached calcined clay, and porous mineral.
U.S. Pat. No. 5,882,396 describes a paper coating composition for preparing a coated paper for ink jet printing including a composite pigment selected from one or more of kaolin, calcined kaolin, dolomite, ground natural calcium carbonate, precipitated calcium carbonate, calcium sulfate, or talc preferably comprising 1–50% by weight coarse pigment and from 99–50% fine pigment of certain prescribed particulate size distributions, and a hydrophilic polymeric adhesive.
U.S. Pat. No. 4,554,181 describes an ink jet recording sheet having a bicomponent cationic recording surface, comprising a substrate having a recording surface containing a cationic polymer that is used in combination with a water-soluble polyvalent metal salt in which the polymer provides the surface with cationic groups for ionically interacting with an anionic dye and insolubilizing it. To the extent the coating compositions contain pigments, the '181 patent does not describe any significance attached to the order of admixing that type of component with the other ingredients, and indicates a single pot mixing procedure for formulating the coating compositions. In addition, the '181 patent only describes the use of non-anionic type binders in coating compositions.
Most cationic polymers are slightly colored in their liquid form ranging from pale orange colors to deep orange-red colors. The inherited colors from cationic polymer ingredients used in paper coating compositions can affect the final coated sheet brightness and shade. Further the use of specific cationic pigments leads to differential reactivity with colored ink jet components which creates good ink hold out but can change the color gamut of the picture that is being printed. It would be desirable to provide coating compositions using reduced levels of cationic polymers, which can still meet performance requirements.
A need exists for less costly pigment alternatives to synthetic silicas for paper coating applications, especially inkjet paper, which provide desirable coating rheology for high solids applications along with uniform printability, including but not limited to color reproduction, print density and ink drying. It would also be desirable if these coating pigments could function with standard paper coating binders and be applied on modern paper machine at high speed.